1. Field of the Invention
The present invention relates to an apparatus and a method for drive controlling a micro machine device including two electrodes opposing each other and a dielectric layer sandwiched therebetween.
2. Description of the Related Art
Recently, application of a micro machine device with a fine structure obtained through micro machine processing technology [also designated as MEMS (Micro Electro Mechanical Systems) or MST (Micro System Technology)] to a radio communication circuit is regarded significant (see Japanese Patent Publication Nos. 2002-84148 and 2002-36197).
Since a micro machine device including a dielectric layer sandwiched between two electrodes functions as a variable capacitor by controlling a driving voltage, a capacitance with a desired value can be obtained in accordance with a control voltage. It is noted that a micro machine device is sometimes designated as a “micro machining device”, a “micro machine element” or an “MEMS electromechanical component”.
On the other hand, in accordance with recent development of mobile communication systems, cellular phones (mobile telephone terminals) and mobile information terminals are now rapidly spreading. For example, in cellular phones, high frequencies of quasi-microwave bands of a 800 MHz through 1.0 GHz band and a 1.5 GHz through 2.0 GHz band are used. In order to cope with such multiple bands employed in cellular phones, it is necessary to provide an amplifier circuit and the like dedicated to a high frequency of each band. This is because it is difficult to construct transmission lines and matching circuits with small loss over a wide band. For overcoming this problem, use of a variable capacitor for enlarging the bandwidth of a matching circuit has been proposed. In particular, a variable capacitor fabricated through the aforementioned micro machine processing technology (hereinafter referred to as a “variable capacitance device”) has a large Q value and can exhibit good characteristics with small loss and small signal distortion (see RF MEMS THEORY, DESIGN AND TECHNOLOGY, WILEY INTERSCIENCE, GABRIEL M. REBEIZ, P 383).
There arises, however, a problem that due to charge-up of a dielectric caused in applying a DC voltage to a variable capacitance device, the initial capacitance cannot be restored even when the DC voltage is eliminated. Therefore, change of the capacitance CP against a control voltage VC exhibits hysteresis, and hence, the reproducibility is poor and a desired capacitance CP cannot be accurately obtained.
Specifically, FIG. 2 shows curves L1 and L2 corresponding to change of the capacitance CP caused by gradually changing the control voltage VC. The control voltage VC is gradually increased from 0 V to 12 V, reduced to 0 V thereafter, further reduced to −12 V, and then increased to 0 V. During this change, the capacitance CP is changed as shown with arrows beside the curves L1 and L2, and the hysteresis is caused. Accordingly, even when the control voltage VC has the same value, the capacitance CP has different values in increasing the control voltage and in reducing the control voltage.
In order to avoid such charge-up, the present applicants have proposed switching drive for inverting (switching) the positive/negative polarities of the control voltage VC in a given short period of time. Specifically, when the switching drive is employed, a positive voltage and a negative voltage are alternately applied to the two electrodes of a variable capacitance device, so as not to cause polarization derived from transfer of space charge within the dielectric layer, and thus, the occurrence of the charge-up is suppressed.
When the switching drive is employed, however, the following problem occurs: In some variable capacitance devices, voltage-capacity characteristics are varied between a positive side and a negative side. Therefore, the value of the capacitance CP may be different between the positive side and the negative side in the switching drive, and hence, a constant capacitance cannot be sometimes attained. Also, since a transmission signal is generally applied to the variable capacitance device even during the switching drive, it is apprehended that the charge-up also may be caused by the transmission signal.
In a variable capacitance device whose characteristics are different between the positive side and the negative side due to the initial variation or the charge-up caused during the use, the value of the capacitance CP is varied in the switching drive. Therefore, when such a variable capacitance device is used in, for example, a matching circuit, it cannot keep necessary characteristics.